Apparatus for preventing overspeed of elastic fluid turbine driven generators

ABSTRACT

An apparatus for preventing overspeed operation of a turbine in response to the output of a turbine-driven generator comprises a control valve for controlling the motive fluid supply to the turbine and an electric control system. The electric control system includes a first switch responsive to the generator output above a first value and a second switch responsive to the generator output below a second value within a predetermined time period and cooperative with the first switch to provide a control signal to accordingly actuate the control valve.

United States Patent Inventor Tetsuzo Sakamoto Yokohama-shi, Japan Appl. No. 774,617

Filed Nov. 12, 1968 Patented Oct. 5, 1971 Assignee Tokyo Shibaura Denki Kabushiki Kaisha Kanagawa-ken, Japan Priority Nov. 14, 1967 Japan APPARATUS FOR PREVENTING OVERSPEED 0F ELASTIC FLUID TURBINE DRIVEN GENERATORS 4 Claims, 4 Drawlng Figs.

Int. Cl 1102b 3/42 Field of Search 317/21, 19; 290/40 [56] References Cited UNITED STATES PATENTS 2,262,560 11/1941 Bryant 290/40 2,404,968 7/1946 Layer 290/40 2,909,671 10/1959 Frederick 290/40 3,156,848 11/1964 Wood 317/19 2,828,448 3/1958 Perkins, Jr. et a1. 317/19 Primary Examiner-]. D. Miller Assistant ExaminerHarry E. Moose, Jr. Attorneys-Robert E. Burns and Emmanuel J. Lobato ABSTRACT: An apparatus for preventing overspeed operation of a turbine in response to the output ofa turbine-driven generator comprises a control valve for controlling the motive fluid supply to the turbine and an electric control system. The electric control system includes a first switch responsive to the generator output above a first value and a second switch responsive to the generator output below a second value within a predetermined time period and cooperative with the first switch to provide a control signal to accordingly actuate the control valve.

32 0 CONTROL TURBINE VALVE GEN T0 INTERCEPT VALVE PATENTEDUCI 5|97I (3,611,039

sum 1 OF 3 i I zs fl /F i I I I I .1 l 24 i I u/2Q l I I 2 0 I l 23 WATTMETER 27 I i J 3O GOVERNOR CONTROL TURBINE VALVE GEN PATENTEDDBT 5l97l 3511.039

SHEET 2 OF 3 PATENTED 0m 5197:

SHEET 3 [IF 3 FIG. 3(0) 25 FIG 3(b) APPARATUS FOR PREVENTING'OVERSPEED F ELASTIC FLUID TURBINE DRIVEN GENERATORS BACKGROUND OF THE INVENTION This invention relates to apparatus for preventing the rotational speedof an elastic fluid turbine driven generator from increasing to abnormal values when the electric load of the generator is rapidly decreased or interrupted.

Abrupt reduction of the load of a turbine generator results in an unbalance between the energy of the motive fluid supplied to the generator and the energy thereof supplied to the associated electric loads. More particularly, the delay in operation of a control valve which is used to control the flow of the motive fluid to the turbine causes surplus in the energy of the motive fluid, thus increasing the rotational speed of the turbine beyond a predetennined value. Such a phenomenon becomes significant with the increase in the capacity of the elastic fluid turbine generator because the density of the motive fluid supplied thereto increases with the capacity of the generator and because the inertia per unit generating capacity of the rotary portion of a steam or gas turbine generator decreases with the capacity.

Generally, speed regulation of a turbine generator is accomplished by detecting the difference between a reference speed and the actual speed by means of a speed governor to open or close a control or adjusting valve in a direction to compensate for the speed difference to control the quantity of steam supplied to the turbine. Usually, the governor is so designed that when the turbine speed departs 4 to 5 percent from its rated speed, the governor operates to vary the quantity of the steam flowing into the turbine from a quantity corresponding to the output at that moment to a quantity corresponding to the noload output. (Such a regulation is referred to as percent regulation of 4 to 5 percent). in a high-capacity generator having small inertia and equipped with such a speed-regulating device, even when the steam-adjusting valve is operated in response to the removal of the full load, for example, of the generator, the speed of the turbine increases substantially due to the steam supplied during an interval of time which is required to close the adjusting valve to a predetermined opening. In other words, the closing operation of the adjusting valve will not be effected until the turbine speed exceeds 4 to 5 percent beyond the rated speed, thus requiring some time delay before the valve is closed to a predetermined opening, and the excess energy supplied during this period causes excessive overspeed of a rotary body of small inertia.

It is preferable to limit such an overspeed to within 1 percent of the rated speed.

In order to effectively prevent overspeed occurring at the time of load interruption or loss, there has been proposed a socalled acceleration relay system wherein the rate of speed increase of a steam turbine is detected to regulate the opening of the adjusting valve before the closing operation of the control valve is effected by the governor. However, with such a system it is difiicult to limit overspeed to 110 percent in a generator which has an inherent property of rapidly increasing its speed as in a generator having a large capacity. This is because before the acceleration relay operates to rapidly decrease the quantity of steam supplied to the turbine, the speed thereof substantially increases since the turbine speed must be actually increased to operate the acceleration relay. Since some time is necessary to operate the adjusting valve in its closing direction, before this valve is controlled by a conventional regulator, the valve must be closed by independent means at the time of load interruption. For this reason, where the condition of steam supplied to the turbine is so adverse that the steam-adjusting valve is required to be opened to a substantial extent under a relatively small electric load of the generator, it takes a longer time to operate the adjusting valve than under the normal condition of steam. As a result, excessive energy will be supplied to the turbine thus making it difficult to decrease the turbine speed to the predetermined value.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a simple apparatus which can control the motive fluid supplied to a turbine so as to prevent overspeed of a turbine generator when the electrical load thereof is rapidly decreased or removed.

Another object of this invention is to provide a new and improved apparatus which commences its operation when the electrical load of a turbine generator decreases beyond a predetermined limit or when an abnonnal unbalance occurs between the opening of a valve means for controlling the motive fluid supplied to a turbine and the electrical load of the generator thereby limiting the speed increase of the turbine generator to a permissible safe value.

According to this invention, there is provided apparatus for preventing an undesirable overspeed condition of an elastic fluid driven turbine generator comprising a load detector for determining whether the electrical load of the generator is above a first predetermined value or below a second predetermined value which is lower than the first value, and wherein the first and second values are selected in such a manner that upon quick reduction in the generator load from the first value to the second value, the generator reaches a dangerous overspeed condition. Means are provided to detect the extent of opening of the control valve, for determining whether the extent of opening of the control valve is larger than a predetermined extent which is substantially equal to the extent of opening of the control valve when an electrical load substantially equal to the first value is applied to the generator under normal operating conditions. Memory means are provided to store for a predetermined interval of time the fact that the load detector has detected the presence of a load exceeding the first value, when the load is decreased from a value exceeding the first value to a value less than the same. The apparatus further includes first means operative when the load detector has detected the fact that the electrical load has a value less than the second value during an interval during which the memory means is storing the presence of a load of more than the first value and second meansoperative when the means for detecting the extent of valve opening detects the fact that the extent of opening of the control valve is more than said predetermined extent and when the load detector detects the fact that the electrical load is less than the second value. Means are also provided to generate a signal for a predetermined interval to cause rapid closure of the control valve when either one or both of first and second means respond and means responsive to the signal are provided for commanding rapid closure of the control valve in preference to the control from the speed governor thus preventing undesirable overspeed.

BRlEF DESCRIPTION OF THE DRAWING The invention can be more fully understood from the following description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a block diagram of one embodiment of this invention;

FIG. 2 is a schematic representation of the entire arrangement of a reheating turbine generator installation to help to understand the principle of this invention and FIGS. 3a and 3b are diagrams; partly in section, of one example of a driving device for a motive fluid-adjusting valve showing same in different operating states.

DESCRIPTION OF THE PREFERRED EMBODlMENT Briefly stated, the apparatus of the present invention operates to render effective the control of valve means by means of a conventional speed-responsive means provided for controlling the speed of a turbine to quickly close said valve means to a predetermined opening when the electrical load of a generator driven by the turbine decreases from a predetermined first value to a second value lower than the first value in a predetermined time interval or when the electrical load of the generator remains at a value less than said second value even when the valve means for controlling the flow of the motive fluid to. the turbine has opened more than a predetermined extent.

Referring now to the accompanying drawing, in a reheating turbine installation shown in FIG. 2, high-temperature and high-pressure steam is supplied to a high-pressure turbine 13 from a boiler 11 through an adjusting or control valve 12. After expansion in turbine 13, the steam is returned to the boiler 11 to be reheated therein, then is supplied to a medium pressure turbine 15 via an intercept valve 14. After expansion and working in the medium-pressure turbine the steam is supplied to a low-pressure turbine 16. Exhaust therefrom is condensed in a condenser 17 and the condensate is returned to boiler 11 by a feedwater pump 19 through a condensate pump 18 and a feedwater heater, not shown.

Accordingly, at the time of quick removal of load, it is impossible to prevent the turbine generator from overspeeding unless the adjusting valve 12 is quickly closed together with the intercept valve 14.

According to the embodiment shown in FIG. 1, a wattmeter 21 is provided to measure the turbine load or the output from a generator 20 directly coupled to the turbine to determine whether the electrical load of the generator is more than a first value or less than a second value which is lower than the first value. The first value is a value at which when the load of turbine generator 20 is rapidly decreased from a value more than said first value to no-load condition, the turbine speed may reach an undesirable dangerous overspeed, for example, more than ll0 percent of the rated speed when the turbine speed is regulated by an ordinary speed regulator, whereas the second value is a value attained by the turbine generator under a condition in which when the load of the generator is rapidly reduced from its rated maximum load to a value less than the second value the turbine generator may reach a dangerous overspeed under the control of the conventional speed governor. In other words, this value corresponds to a value wherein when the turbine generator is operating at a load below the first value, perfect loss of such a load does not result in any dangerous overspeed under control of the conventional speed governor and so long as the reduced load is more than the second value, any dangerous condition will not be created under control of the conventional speed governor. These first and second values are inherent to a system including the turbine, the generator coupled thereto and the speed governor system, and are selected at suitable values dependent upon the system. For the purpose of description, it is herein assumed that the first value equals 75 percent whereas the second value equals percent of the rated value. wattmeter 21 is provided with a movable contact 211 arranged to be closed when the load exceeds the first value or is more than 75 percent of the rating and a movable contact 212 arranged to be closed when the load is less than the second value or less than 25 percent of the rating. It is clear that these contacts are not closed at the same time.

As shown in FIG. 1, the contact 211 of the wattmeter is connected across conductors 22 and 23 of a control source in series with a time delay reset-type relay 24 acting as a timer of the off-delay characteristic. When energized by the closure of the contact 211, relay 24 operates to instantly close a movable contact 241 which is designed to be opened for a predetermined time interval T, after deenergization of the relay 24 caused by the opening of the contact 211. Contact 241 is connected across source conductors 22 and 23 in series with contact 212 of the wattmeter 21 and a relay 25 operates to generate a signal to rapidly decrease the energy supplied to the turbine. As a result, when the load is reduced from a condition in which the generator is operating at a load of more than 75 percent (under this condition contact 211 is closed to energize relay 24 thus closing contact 241) to a load condition of less than 25 percent during said time interval T,, since the contact 212 is closed while contact 241 is closed, the relay 25 will be energized from source conductors 22 and 23. If the load is gradually reduced so that the time interval between an instant at which the load is decreased to a value less than 75 percent or the contact 211 is opened and an instant at which the load is decreased to a value less than 25 percent or contact 212 is closed is longer than period T,, contact 241 will be opened before contact 2l2.is closed so that the relay 25 will not be energized by contacts 241 and 212. Under these conditions it is not necessary to quickly close adjusting valve 12 and intercept valve 14 because an ordinary speed governor will suffice. As will be noted from the foregoing description the off-delay timer 24 acts as a memory means to store for a predetermined time interval T, the load condition before its reduction, said interval T, being selected in accordance with the system such that energization of the relay 25 can be prevented where it is not necessary to rapidly close the adjusting valve, in the same manner as at the first and second values.

Under adverse steam condition, since the adjusting valve is substantially opened even at loads of less than 75 percent, there is a likelihood that the turbine may attain an overspeed of more than 1 10 percent when the load is abruptly decreased to a value less than 25 percent. For this reason, in order to energize relay 25 under these condition, there is provided in parallel with contact 241 a movable contact 26 which is closed when the control valve is opened beyond a predetermined extent. Contact 26 may be a contact of a limit switch 8, for example, utilized to mechanically detect the stroke of the valve member of the control valve. Thus, for example, contact 26 is arranged to be closed when the adjusting valve is opened beyond a predetermined opening, for example 75 percent, at which the electrical load is equal to or slightly less than 75 percent in normal steam condition.

A suitable timer 27 is connected in parallel with the relay 25 to determine the period of energization thereof and has a contact 271 connected in parallel with the contacts 241 and 26 and is arranged to be instantly closed when the timer 27 is energized but opened with a predetermined time delay T,. This time delay or interval T is determined to be substantially equal to the time interval in which the rotational speed of the turbine generator increases to a speed at the time of load loss, and the speed thus increased becomes settled to a substantially rated speed, or, the time interval required for the increased rotational speed at the time of the load loss to return to its rated speed. This interval can be determined by mathematical calculations for each turbine.

In FIG. 1, the turbine generator is generally shown as a block 20 while a speed governor mechanism which compares the turbine speed with a given reference speed or signal r to control valve 32 through a hydraulic servomechanism 31 is shown by blocks 29 and 30.

While a control circuit utilizing movable mechanical contacts is illustrated in FIG. 1, it will be clear that the control circuit may be substituted by a solid-state circuit utilizing transistors and the like. In the latter circuit, wattmeter 21 may be replaced by a static-type meter utilizing the Hall effect and the means for detecting the extent of opening of the valve by a static proximity switch or by a suitable detector comprising a combination of a mechanical element and a nonmechanical element.

As shown in FIG. 3, the relay 25 operates to provide a com- I mand signal to a liquid pressure driving mechanism for the adjusting or control valve 32 at times when overspeed conditions may occur.

More particularly, as shown in FIG. 3, three spaced-apart lands 33a, 33b and 330 are mounted on a stem 34 to be slidable in a cylinder 35 and a connecting cylinder 36. The lands 33a and 33b and the cylinder 35 comprise a conventional turbine control device under control of a speed governor. The uppermost land 330 is biased downwardly by means of a spring 37 contained in a cylinder 36. Liquid under pressure is applied to the lower surface of land 33c through a three-way valve 38 which is constructed to releasefluid pressure from the lower surface of land 33c when relay 25 is energized. Consequently,

when relay is not energized, fluid pressure is applied to the lower surface of land 33c contained in connecting cylinder 36 to urge upwardly the land 33c until the upper end of stem 34 comes into engagement with the upper end of cylinder 36, as shown in FIG. 30. Under this state, cylinder 36 and stem 34 move in unison. Pivotally connected to the connecting cylinder 36 is a link 41 with its left-hand end connected to a governor, not shown. Right-hand end of link 41 is connected to a piston rod of a servomotor 39 via a link 42 and a lever 43 fulcrumed at an intermediate point, thus providing restoring action for the servomotor. Furthermore, the movement of the piston rod of servomotor 39 is utilized to open or close control valve 32 via a link 44 pivotally mounted on a stationary member at its right-hand end. Thus, movement of the lefthand end of link 41 shown by double head arrows results in the opening and closing operation of control valve 32.

Upon energization of the relay 25, the fluid pressure is released from the lower surface of land 33c thus permitting the lands to descend to the position shown in FIG. 3b under the bias of the spring 37. As a result, stem 34 is lowered irrespective to the mechanical signal applied to the left-hand end of lever 41 by the speed governor, thus discharging the liquid in servomotor 39, this in turn causing rapid closure of control valve 32. The vertical stroke of the land 330 in cylinder 36 is determined to a suitable value in accordance with the downward stroke of stem 34 of pilot valve caused by total removal of the load and also by taking into consideration the time interval T during which relay 25 is energized.

The operation of the apparatus is as follows: Assume first that the turbine generator is operating at a load ranging from 75 to 100 percent of the rated load. Then movable contacts 211, 241 and 26 are closed, but contact 212 is opened. Thus auxiliary relay 25 is deenergized so that three-way valve 38 is operated to a position in which fluid under pressure is supplied to the underside of land 33c to force the upper end of the stem 34 into engagement with the upper end of cylinder 36 thus integrally uniting the cylinder 36 and stem 34. Under this condition, the stem 34 and lands carried thereby are placed under control of an ordinary speed control system so as to drive servomotor 39 in accordance with the variation of the load to maintain the speed at a definite value.

Upon rapid reduction of the load, the output of the generator thereafter immediately decreases to a value less than 75 percent in a very short time so that movable contact 211 is opened. However, the movable contact 241 is maintained in its closed condition for the time interval T by timer 24. Therefore, when the speed of the turbine generator decreases rapidly, and when the load decreases to a value less than 25 percent upon interruption of the load, contact 212 will be closed during said period in which contact 241 is maintained closed. Then relay 25 is energized and its holding circuit is completed through contact 271 of the relay 27 for a time interval T 2 during which three-way valve 38 operates to switch the fluid conduits to relieve fluid pressure from the lower surface of land 33c. As a result, land 33c is urged downwardly by spring 37 together with lands 33a and 33b of pilot valve to connect the cylinder of the servomotor 39 to the low-pressure drain as shown by a fine line with arrowheads in FIG. 3b. Thus the piston of the servomotor is rapidly lowered by spring 40 to rapidly close adjusting valve 32.

When the load is rapidly decreased, if the load is not immediately reduced to a value less than 25 percent within said prescribed time interval, for example several seconds, the adjusting valve 32 would not be rapidly closed.

Thus, when the steam condition is unsatisfactory and the valve 32 is opened so widely that the movable contact 26 is closed even though the output is below 75 percent of the rated value, the contact 241 is thus opened and upon rapid reduction of the load, it is therefore necessary to rapidly close the valve because of its large extent of opening. In such a case, since the contact 26 is closed, closure of the contact 212 causes energization of the relays 25 and 27 to rapidly close the valve before the opening of the contact 26, in other words, be-

fore the closing operation of the valve is initiated by the conventional speed governor. Since contact 271 connected in parallel with contact 26 is maintained in its closed position for interval T relay 25 is maintained in its energized state, thus effecting rapid closure of the valve.

Thus, according to this invention, where a condition is created which tends to cause overspeed of the turbine upon removal of the load, the adjusting valve is positively and rapidly closed to prevent overspeed.

While in FIG. 1, only an adjusting valve has been shown as the controlled part, it should be understood that an intercept valve or other parts may also be controlled in the same manner.

It is to be understood that means to associate a relay with a driving mechanism for an adjusting valve is not necessarily limited to that illustrated in the drawing and that any suitable mechanism can be used.

Further, in addition to a steam turbine generator, this invention is also applicable to a gas turbine generator provided that the fuel injection valve can be closed to an extent which assures minimum flow of fuel required for continuous or no-load operation of the gas turbine by means of a relay which responds to load interruption.

lclaim:

1. In an elastic fluid turbine driven generator system having a turbine-driven generator, a turbine rotationally driven by motive fluid and connected to rotationally drive said generator, valve means including a control valve for controlling the flow of motive fluid supplied to said turbine, and speedresponsive means to control said valve means to maintain the speed of said turbine at a predetermined constant value, an improvement in an apparatus for preventing overspeed of said turbine generator comprising: a load detector operative to determine whether the electrical load of said generator is above a first predetermined value or below a second predetermined value which is lower than said first value, said first and second values being selected such that upon quick reduction in the electrical load of said generator from said first value to said second value said generator reaches an undesirable overspeed; means to detect the extent of opening of said control valve for determining whether the extent of opening of said control valve is larger than a predetermined extent of opening which is substantially equal to the extent of opening of said control valve when an electrical load substantially equal to said first value is applied to said generator under normal condition; memory means to store for a predetermined interval of time the fact that said load detector has detected the presence of a load exceeding said first value, when the electrical load is decreased from a value exceeding said first value to a value less than the same; first means for providing a first signal when said load detector has detected the fact that the electrical load has a value less than said second value during an interval during which said memory means is storing the presence of a load of more than said first value; second means for providing a second signal when said means for detecting the extent of opening of said control valve detects the fact that said extent is more than said predetermined extent of opening and when said load detector detects the fact that the electrical load is less than said second value; means to generate a command signal for a predetermined interval to cause rapid closure of said control valve in response to either one or both of said first and second signals and means responsive to said command signals to rapidly close said control valve in preference to the control by said speed-responsive means thus preventing undesirable overspeed.

2. A steam turbine arrangement for driving a generator supplying electric power to an electric load, said steam turbine ar rangement including valve means to control steam applied to said turbine, and a speed governor which controls said valve means through a first fluid pressure servomechanism comprising a pilot valve and a hydraulic relay responsive to the fluid pressure from said pilot valve to actuate said valve means, an improvement in an apparatus for preventing overspeed of said turbine comprising; means including a wattmeter for detecting the effective electric load of said generator to provide a first output when said load is above a first predetermined value and a second output when said load is less than a second predetermined value, said first and second values being set such that said governor reaches an undesirable overspeed when said load rapidly decreases from a value above said first value to a value less than said second value; means to detect valve opening of said valve means to provide a third output when the extent of opening of said valve means is above a predetermined extent which is substantially equal to the extent of opening of said valve means when said electric load is nearly equal to said first valve under normal steam conditions; a memory device responsive to said first output to provide a fourth output having an ofi-delay characteristic; means to provide a fifth output when said second and fourth outputs exists simultaneously; means to provide a sixth output when said second and third outputs exist simultaneously; means responsive to either one of said fifth and sixth outputs to provide for a predetermined time interval a closing signal to effect rapid closing of said valve means; and a second fluid-operated servomechanism responsive to said closing signal to cause said first servomechanism to override said governor to rapidly close said valve means thus preventing undesirable overspeed.

3. The apparatus as claimed in claim 2, wherein said second fluid-operated servomechanism is included in a mechanism transmitting a signal from said governor to said pilot valve and comprises a cylinder connected to said transmitting mechanism and a spring-biased land contained in said cylinder and connected to said pilot valve.

4. In combination: a generator rotationally driven to develop an electrical output and having a predetermined overspeed condition; a fluid-driven turbine connected to said generator operative in response to motive fluid applied thereto to rotationally drive said generator; valve means for controlling the supply of motive fluid to said turbine in accordance with the rotational speed thereof; detecting means for detecting when the electrical output of said generator decreases from a first predetermined value to a second predetermined value within a time period T representative of an overspeed condition of said generator and providing a corresponding output signal, said detecting means comprising first normally open electric switching means operable to a closed position whenever said generator output exceeds said first predetermined value and for said time period T after said output falls below said first predetermined value, second normally open electric switching means operable to a closed position whenever said generator output falls below said second predetermined value for said time period T,, and electric circuit means including said first and second switching means for providing said output signal whenever a portion of said time period T coincides with a portion of said time period T,; and means responsive to said output signal for overriding said valve means to effect a reduction in speed of said turbine for a predetermined time period T 

1. In an elastic fluid turbine driven generator system having a turbine-driven generator, a turbine rotationally driven by motive fluiD and connected to rotationally drive said generator, valve means including a control valve for controlling the flow of motive fluid supplied to said turbine, and speed-responsive means to control said valve means to maintain the speed of said turbine at a predetermined constant value, an improvement in an apparatus for preventing overspeed of said turbine generator comprising: a load detector operative to determine whether the electrical load of said generator is above a first predetermined value or below a second predetermined value which is lower than said first value, said first and second values being selected such that upon quick reduction in the electrical load of said generator from said first value to said second value said generator reaches an undesirable overspeed; means to detect the extent of opening of said control valve for determining whether the extent of opening of said control valve is larger than a predetermined extent of opening which is substantially equal to the extent of opening of said control valve when an electrical load substantially equal to said first value is applied to said generator under normal condition; memory means to store for a predetermined interval of time the fact that said load detector has detected the presence of a load exceeding said first value, when the electrical load is decreased from a value exceeding said first value to a value less than the same; first means for providing a first signal when said load detector has detected the fact that the electrical load has a value less than said second value during an interval during which said memory means is storing the presence of a load of more than said first value; second means for providing a second signal when said means for detecting the extent of opening of said control valve detects the fact that said extent is more than said predetermined extent of opening and when said load detector detects the fact that the electrical load is less than said second value; means to generate a command signal for a predetermined interval to cause rapid closure of said control valve in response to either one or both of said first and second signals and means responsive to said command signals to rapidly close said control valve in preference to the control by said speed-responsive means thus preventing undesirable overspeed.
 2. A steam turbine arrangement for driving a generator supplying electric power to an electric load, said steam turbine arrangement including valve means to control steam applied to said turbine, and a speed governor which controls said valve means through a first fluid pressure servomechanism comprising a pilot valve and a hydraulic relay responsive to the fluid pressure from said pilot valve to actuate said valve means, an improvement in an apparatus for preventing overspeed of said turbine comprising; means including a wattmeter for detecting the effective electric load of said generator to provide a first output when said load is above a first predetermined value and a second output when said load is less than a second predetermined value, said first and second values being set such that said governor reaches an undesirable overspeed when said load rapidly decreases from a value above said first value to a value less than said second value; means to detect valve opening of said valve means to provide a third output when the extent of opening of said valve means is above a predetermined extent which is substantially equal to the extent of opening of said valve means when said electric load is nearly equal to said first valve under normal steam conditions; a memory device responsive to said first output to provide a fourth output having an off-delay characteristic; means to provide a fifth output when said second and fourth outputs exists simultaneously; means to provide a sixth output when said second and third outputs exist simultaneously; means responsive to either one of said fifth and sixth outputs to provide for a predetermined time interval a closing signal to Effect rapid closing of said valve means; and a second fluid-operated servomechanism responsive to said closing signal to cause said first servomechanism to override said governor to rapidly close said valve means thus preventing undesirable overspeed.
 3. The apparatus as claimed in claim 2, wherein said second fluid-operated servomechanism is included in a mechanism transmitting a signal from said governor to said pilot valve and comprises a cylinder connected to said transmitting mechanism and a spring-biased land contained in said cylinder and connected to said pilot valve.
 4. In combination: a generator rotationally driven to develop an electrical output and having a predetermined overspeed condition; a fluid-driven turbine connected to said generator operative in response to motive fluid applied thereto to rotationally drive said generator; valve means for controlling the supply of motive fluid to said turbine in accordance with the rotational speed thereof; detecting means for detecting when the electrical output of said generator decreases from a first predetermined value to a second predetermined value within a time period T1 representative of an overspeed condition of said generator and providing a corresponding output signal, said detecting means comprising first normally open electric switching means operable to a closed position whenever said generator output exceeds said first predetermined value and for said time period T1 after said output falls below said first predetermined value, second normally open electric switching means operable to a closed position whenever said generator output falls below said second predetermined value for said time period T2, and electric circuit means including said first and second switching means for providing said output signal whenever a portion of said time period T1 coincides with a portion of said time period T2; and means responsive to said output signal for overriding said valve means to effect a reduction in speed of said turbine for a predetermined time period T2. 